Multi-polarization, low frequency ultra-wideband synthetic aperture radar (UWB SAR) is one of the most important aspects of the radar technology. The low frequency, UWB and large processing angle result that the common SAR polarimetric calibration cannot be used. In this paper, with the system echo model, considered the electromagnetic scattering characteristics of the reference calibrator, the polarimetric calibration for the system is proposed. The computer simulation is used to validate the method.